High-Speed Circuit Board Signal Integrity

262 Pages·2004·2.53 MB·English

by  Stephen C. Thierauf

High-Speed Circuit Board
Signal Integrity

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turn to the back of this book.

High-Speed Circuit Board
Signal Integrity

Stephen C. Thierauf

Artech House, Inc.
Boston • London
www.artechhouse.com

Library of Congress Cataloguing-in-Publication Data

A catalog record for this book is available from the U.S. Library of Congress.

British Library Cataloguing in Publication Data

A catalog record for this book is available from the British Library.

Cover design by Igor Valdman

© 2004 ARTECH HOUSE, INC.
685 Canton Street
Norwood, MA 02062

All rights reserved. Printed and bound in the United States of America. No part of this book
may be reproduced or utilized in any form or by any means, electronic or mechanical, includ-
ing photocopying, recording, or by any information storage and retrieval system, without
permission in writing from the publisher.
All terms mentioned in this book that are known to be trademarks or service marks have
been appropriately capitalized. Artech House cannot attest to the accuracy of this informa-
tion. Use of a term in this book should not be regarded as affecting the validity of any trade-
mark or service mark.

International Standard Book Number: 1-58053-131-8

10 9 8 7 6 5 4 3 2 1

To Ann, Christopher, and Kevin

.

Contents

Preface
xiii

CHAPTER 1
Characteristics and Construction of Printed Wiring Boards
1

1.1
Introduction
1
1.2
Unit System
1
1.3
PWB Construction
2
1.3.1
Resins
3
1.3.2
Alternate Resin Systems
3
1.3.3
Reinforcements
5
1.3.4
Variability in Building Stackups
6
1.3.5
Mixing Laminate Types
7
1.4
PWB Traces
7
1.4.1
Copper Cladding
8
1.4.2
Copper Weights and Thickness
9
1.4.3
Plating the Surface Traces
9
1.4.4
Trace Etch Shape Effects
9
1.5
Vias
10
1.5.1
Via Aspect Ratio
13
1.6
Surface Finishes and Solder Mask
14
1.7
Summary
14
References
15

CHAPTER 2
Resistance of Etched Conductors
17

2.1
Introduction
17
2.2
Resistance at Low Frequencies
17
2.3
Loop Resistance and the Proximity Effect
20
2.3.1
Resistance Matrix
21
2.3.2
Proximity Effect
22
2.4
Resistance Increase with Frequency: Skin Effect
24
2.5
Hand Calculations of Frequency-Dependent Resistance
27
2.5.1
Return Path Resistance
28
2.5.2
Conductor Resistance
28
2.5.3
Total Loop Resistance
29
2.6
Resistance Increase Due to Surface Roughness
29
2.7
Summary
30

vii

References
30

CHAPTER 3
Capacitance of Etched Conductors
31

3.1
Introduction
31
3.2
Capacitance and Charge
31
3.2.1
Dielectric Constant
32
3.3
Parallel Plate Capacitor
33
3.4
Self and Mutual Capacitance
35
3.5
Capacitance Matrix
37
3.6
Dielectric Losses
39
3.6.1
Reactance and Displacement Current
40
3.6.2
Loss Tangent
40
3.6.3
Calculating Loss Tangent and Conductance G
41
3.7
Environmental Effects on Laminate εr and Loss Tangent
43
3.7.1
Temperature Effects
44
3.7.2
Moisture Effects
44
3.8
Summary
45
References
45

CHAPTER 4
Inductance of Etched Conductors
47

4.1
Introduction
47
4.2
Field Theory
47
4.2.1
Permeability
48
4.2.2
Inductance
48
4.2.3
Internal and External Inductance
49
4.2.4
Partial Inductance
49
4.2.5
Reciprocity Principal and Transverse Electromagnetic Mode
50
4.3
Circuit Behavior of Inductance
51
4.3.1
Inductive Voltage Drop
53
4.3.2
Inductive Reactance
54
4.4
Inductance Matrix
55
4.4.1
Using the Reciprocity Principle to Obtain the
Inductance Matrix from a Capacitance Matrix
55
4.5
Mutual Inductance
55
4.5.1
Coupling Coefficient
56
4.5.2
Beneficial Effects of Mutual Inductance
57
4.5.3
Deleterious Effects of Mutual Inductance
59
4.6
Hand Calculations for Inductance
60
4.6.1
Inductance of a Wire Above a Return Plane
60
4.6.2
Inductance of Side-by-Side Wires
61
4.6.3
Inductance of Parallel Plates
61
4.6.4
Inductance of Microstrip
63
4.6.5
Inductance of Stripline
63
4.7
Summary
64
References
65

viii
Contents

CHAPTER 5

Transmission Lines
67

5.1
Introduction
67
5.2
General Circuit Model of a Lossy Transmission Line
67
5.2.1
Relationship Between ωL and R
70
5.2.2
Relationship Between ωC and G
70
5.3
Impedance
71
5.3.1
Calculating Impedance
72
5.4
Traveling Waves
73
5.4.1
Propagation Constant
74
5.4.2
Phase Shift, Delay, and Wavelength
75
5.4.3
Phase Constant at High Frequencies When R and G Are Small
78
5.4.4
Attenuation
79
5.4.5
Neper and Decibel Conversion
80
5.5
Summary and Worked Examples
82
References
86

CHAPTER 6
Return Paths and Power Supply Decoupling
87

6.1
Introduction
87
6.2
Proper Return Paths
87
6.2.1
Return Paths of Ground-Referenced Signals
89
6.2.2
Stripline
90
6.3
Stripline Routed Between Power and Ground Planes
90
6.3.1
When Power Plane Voltage Is the Same as Signal Voltage
90
6.3.2
When Power Plane Voltage Differs from Signal Voltage
93
6.3.3
Power System Inductance
94
6.4
Split Planes, Motes, and Layer Changes
95
6.4.1
Motes
95
6.4.2
Layer Changes
98
6.5
Connectors and Dense Pin Fields
98
6.5.1
Plane Perforation
99
6.5.2
Antipads
99
6.5.3
Nonfunctional Pads
102
6.5.4
Guidelines for Routing Through Dense Pin Fields
103
6.6
Power Supply Bypass/Decoupling Capacitance
105
6.6.1
Power Supply Integrity
106
6.6.2
Distributed Power Supply Interconnect Model
110
6.7
Connecting to Decoupling Capacitors
112
6.7.1
Via Inductance
112
6.8
Summary
114
References
115

CHAPTER 7
Serial Communication, Loss, and Equalization
117

7.1
Introduction
117
7.2
Harmonic Contents of a Data Stream
117

Contents
ix

7.2.1
Line Spectra
119
7.2.2
Combining Harmonics to Create a Pulse
120
7.2.3
The Fourier Integral
122
7.2.4
Rectangular Pulses with Nonzero Rise Times
123
7.3
Line Codes
125
7.4
Bit Rate and Data Rate
126
7.5
Block Codes Used in Serial Transmission
128
7.6
ISI
130
7.6.1
Dispersion
130
7.6.2
Lone 1-Bit Pattern
131
7.7
Eye Diagrams
132
7.8
Equalization and Preemphasis
134
7.8.1
Preemphasis
134
7.8.2
Passive Equalizers
137
7.8.3
Passive RC Equalizer
139
7.9
DC-Blocking Capacitors
140
7.9.1
Calculating the Coupling Capacitor Value
142
7.10
Summary
145
References
146

CHAPTER 8
Single-Ended and Differential Signaling and Crosstalk
149

8.1
Introduction
149
8.2
Odd and Even Modes
149
8.2.1
Circuit Description of Odd and Even Modes
150
8.2.2
Coupling Coefficient
153
8.2.3
Stripline and Microstrip Odd- and Even-Mode Timing
155
8.2.4
Effects of Spacing on Impedance
157
8.3
Multiconductor Transmission Lines
158
8.3.1
Bus Segmentation for Simulation Purposes
159
8.3.2
Switching Behavior of a Wide Bus
160
8.3.3
Simulation Results for Loosely Coupled Lines
161
8.3.4
Simulation Results for Tightly Coupled Lines
162
8.3.5
Data-Dependent Timing Jitter in Multiconductor
Transmission Lines
164
8.4
Differential Signaling, Termination, and Layout Rules
165
8.4.1
Differential Signals and Noise Rejection
165
8.4.2
Differential Impedance and Termination
166
8.4.3
Reflection Coefficient and Return Loss
170
8.4.4
PWB Layout Rules When Routing Differential Pairs
172
8.5
Crosstalk
173
8.5.1
Coupled-Line Circuit Model
175
8.5.2
NEXT and FEXT Coupling Factors
177
8.5.3
Using Kb to Predict NEXT
178
8.5.4
Using Kf to Predict FEXT
179
8.5.5
Guard Traces
179
8.5.6
Crosstalk Worked Example
180

x
Contents

8.5.7
Crosstalk Summary
182
8.6
Summary
182
References
183

CHAPTER 9
Characteristics of Printed Wiring Stripline and Microstrips
185

9.1
Introduction
185
9.2
Stripline
185
9.2.1
Time of Flight
186
9.2.2
Impedance Relationship Between Trace Width,
Thickness, and Plate Spacing
187
9.2.3
Mask Biasing to Obtain a Specific Impedance
189
9.2.4
Hand Calculation of Zo
189
9.2.5
Stripline Fabrication
191
9.3
Microstrip
193
9.3.1
Exposed Microstrip
194
9.3.2
Solder Mask and Embedded Microstrip
196
9.4
Losses in Stripline and Microstrip
197
9.4.1
Dielectric Loss
199
9.4.2
Conductor Loss
199
9.5
Microstrip and Stripline Differential Pairs
201
9.5.1
Broadside Coupled Stripline
201
9.5.2
Edge-Coupled Stripline
204
9.5.3
Edge-Coupled Microstrip
205
9.6
Summary
206
References
207

CHAPTER 10
Surface Mount Capacitors
209

10.1
Introduction
209
10.2
Ceramic Surface Mount Capacitors
209
10.2.1
Dielectric Temperature Characteristics Classification
209
10.2.2
Body Size Coding
211
10.2.3
Frequency Response
212
10.2.4
Inductive Effects: ESL
214
10.2.5
Dielectric and Conductor Losses: ESR
215
10.2.6
Leakage Currents: Insulation Resistance
218
10.2.7
Electrical Model
219
10.2.8
MLCC Capacitor Aging
220
10.2.9
Capacitance Change with DC Bias and Frequency
221
10.2.10
MLCC Usage Guidelines
222
10.3
SMT Tantalum Capacitors
223
10.3.1
Body Size Coding
223
10.3.2
Frequency Response
224
10.3.3
Electrical Model
225
10.3.4
Aging
225
10.3.5
Effects of DC Bias, Temperature, and Relative Humidity
225

Contents
xi

10.3.6
Failure of Tantalum Capacitors
226
10.3.7
ESR and Self Heating: Voltage and Temperature Derating
227
10.3.8
Usage Guidelines
227
10.4
Replacing Tantalum with High-Valued Ceramic Capacitors
228
References
230

Appendix: Conversion Factors
231

About the Author
233

Index
235

xii
Contents

Preface

This is a book for engineers designing high-speed circuit boards. To the signal integ-
rity engineer, this book will be a handbook of formulas and terminology as well as a
refresher of basic electrostatic and electromagnetic principals. The high-speed cir-
cuit designer will find this book an easy entry into the electromagnetics and physics
of high-speed signaling. It introduces concepts fundamental to high-speed signaling,
such as lossy transmission line behavior, skin effect, and the characteristics of lami-
nates and surface mount capacitors. The focus throughout is on the effects of dielec-
tric and conductor loss on signal quality, with a particular emphasis on serial
differential signaling. Coupling between transmission lines (especially in the context
of crosstalk and odd/even modes) is discussed. Besides being useful in serial signal-
ing, this has application to multiconductor busses.
Reflections on transmission lines are only superficially covered in this text. This
topic has been extensively covered in the literature, and the reader of this book is
assumed to be familiar with the creation and mitigation of reflections on transmis-
sion lines. However, the proper routing and termination of differential pairs has not
been as well covered in the literature and so is discussed in Chapter 8.
Similarly, power supply decoupling has been thoroughly discussed elsewhere,
so the coverage in this book is brief. Instead, the focus here is on managing return
paths (something often not well covered) and the electrical characteristics and
behavior of capacitors. The material in Chapter 10 is a concise catalog of essential
electrical characteristics of discrete capacitors, with a focus on surface mount
technology.
The fundamentals of resistance, capacitance, inductance, and loss calculations
presented in Chapters 2–5 are illustrated with practical worked examples that may
be used as templates to solve similar problems.
Many simple formulas are presented to allow hand calculation of resistance,
capacitance, inductance, and impedance. These types of calculations are helpful in
developing intuition and in approximating beforehand the results to be expected
from field solving software, circuit simulation tools, and laboratory measurements.
Extensive references are given at the end of each chapter, providing the inter-
ested reader the opportunity to dig deeper. The references intentionally span classic,
older works (some of which were written in the 1950s, but most of the older ones
are from the 1960s and 1970s) as well as modern works. The older references are
valuable, as they are the original works often cited by others, sometimes without the
proper context. Although long out of print, the selected older references are gener-
ally available secondhand and are worthy additions to the signal integrity engineer’s
library. Particular mention is made of Johnson’s Transmission Lines and Networks

xiii

(published in 1950, referenced in Chapter 2) and Skilling’s Electrical Engineering
Circuits (1965, see the references in Chapter 3) and Transient Electric Currents
(1952). These books are succinct and in my view remain unmatched. Miner’s Lines
and Electric Fields for Engineers (1996, first referenced in Chapter 3) is the one elec-
tromagnetics textbook every signal integrity engineer should have in his or her
library.
I’m indebted to my friends and coworkers for their support, encouragement,
and help during the creation of this book. Special mention must be made of the assis-
tance, perspective, and advice provided by my colleagues Jeff Cooper, Ernie Grella,
and Tim Haynes. Special thanks also goes to Fahrudin Alagic for his many months
of precise laboratory measurements that support the material appearing in Chapters
5–7. I’m grateful to all of those who suffered through early versions of the manu-
script for their constructive remarks. I’m also obliged to the anonymous reviewer for
his insightful comments. All of these comments were most helpful and have resulted
in an improved text. Of course, any inaccuracies or errors that made it into the text
are my doing and in no way reflect on the reviewers.
Finally, I’m especially grateful to my wife Ann for her understanding, patience,
encouragement, and unflagging support throughout the many long hours it took to
create this work. This book would not have been possible without her.

xiv
Preface